The invention relates to a circuit arrangement comprising a hoist controller of a hoist, a pendant control switch having an emergency stop switch, and a control line which electrically connects the hoist controller to the pendant control switch and which has a control voltage applied thereto.
An emergency shutdown switch (also referred to as emergency shutdown) and an emergency stop switch (also referred to as emergency stop) are specific switches on machines, vehicles and installations, in order to rapidly put them in a safe state when a hazardous situation arises or to avert such a hazardous situation. Depending upon the field of application, various strategies are pursued, in the simplest case the activation is followed by an interruption in the supply of current. These switches have a red actuating element on a yellow base. Mushroom buttons are the most common design, in the case of large installations such as e.g. conveying systems trigger line switches (pull cord switches) are also typical.
Switching devices or controls which satisfy the necessary requirements of reliability and further safety aspects are typically used together with emergency shutdown or emergency stop switches, and therefore for instance after an emergency stop a machine must not start up again automatically and in any case all of the components must not be switched off. Depending upon technical conditions, emergency stop switches are classified into stop categories (pursuant to EN 60204-1: safety of machines—electrical equipment of machines—general requirements). A distinction is made between stop categories 0, 1 and 2. In stop category 0, the supply of energy to the drive elements is fully disrupted, if the sudden switching-off of the energy does not cause a hazard. In the case of stop category 1, shut-down is controlled. The machine is put in a safe state and only then is the energy to the drive elements fully disrupted. This is practical if devices such as clamps or brakes require energy to function. In the case of stop category 2, the machine is put in a safe state, but the energy is not disrupted. This category should only be used if from a technical point of view it is not possible to cut off the energy safely. For example, in the case of a crane having a lifting magnet, switching off the voltage at the magnet would cause the load to fall.
In contrast to the emergency shutdown, in the case of the emergency stop it is not necessary for the entire machine to have its energy supply cut off, but rather when a dangerous situation is detected a drive must be stopped in order to protect against any danger.
Therefore, problems are caused by a short-circuit of the emergency shutdown switch or emergency stop switch itself or a short-circuit (also referred to as cross-circuit or interwire short) in the associated wire lines. Short-circuits of this type would short-out the emergency shutdown switch or emergency stop switch and thus prevent a protective function in the event of an emergency. Therefore, it is important that a short-circuit is discovered in good time and in a reliable manner.
Some known options for reducing the probability of cross-circuits or interwire short-circuits are fixed, protected and/or separate routing of the wire lines (e.g. by routing them in cable channels or armoured conduits or the use of different sheathed lines). It is thus possible to assume a fault exclusion for short-circuits between the conductors (EN ISO 13849-2-appendix D.5.2).
The aforementioned measures are not practical for hoists having a pendant switch: the control line between the control switch suspended at operator-height and the hoist or crane—typically at the height of the hall—is subjected to strong tensile forces and vigorous movements as a consequence of usage, so that in this case a fault exclusion for short-circuits between the conductors is not readily possible in this manner.
Further known devices, such as e.g. by the company Pilz, have a consumer, e.g. a resistor in series with the switch, so that a lowering of the voltage occurs, the sources of which are monitored for discovering a cross-circuit. Alternatively, it is also known from the company Pilz to switch off the voltage supply to the two channels of a two-channel switch in staggered fashion for several milliseconds and to check the inputs to see if a voltage is present, which indicates a cross-circuit.
A circuit arrangement for the safe switching-off of a machine installation is already known from German laid-open specification DE 199 62 497 A1. The circuit arrangement includes essentially a signal unit, such as e.g. a light barrier or an emergency shutdown button which is connected to a safety switching device via an electrical line. The safety switching device monitors the operating state of the machine installation. This includes e.g. state variables of the machine installation, such as the position of the emergency shutdown button, the open and closed position of a protection door or the state signal of a light barrier. A safe state is achieved in this case by switching off the supply of current to the machine installation. In order to ensure that the safety switching device can perform a self-test of its signal paths independently of the signal unit used, it is provided that a periodic clock signal is superimposed upon the output signal of the signal unit within the safety switching device between an input stage and an evaluation and switch-off unit. In the evaluation and switch-off unit, this clock signal can then be filtered out and defective cross-circuits can be established within the safety switching device.
European patent application EP 1 380 533 A1 discloses a circuit arrangement for controlling a crane. The circuit arrangement includes a crane control, which is based upon a bidirectional data bus, and a pendant control switch having an emergency stop switch. The pendant control switch is electrically connected to the crane control via a control line.